program Project2;

{$APPTYPE CONSOLE}

uses
  SysUtils;

var
  n, a, i: integer;

begin
  readln (n);
  a:= 0;
  if (n mod 2 = 0) then
    for i:= 2 to n do
      if (i mod 2 = 0) then
        a:= a+i;
  if (n mod 2 = 1) then
    for i:= 3 to n do
      if (i mod 2 = 1) then
        a:= a+i;
  if (n<2) then
    a:= 0;
  write (a);
  readln;
end.
